This invention relates to a process for removing alkaline catalysts from polyether polyols produced by base-catalyzed alkoxylation reactions.
In practice, polyalkylene oxide polyethers are primarily used as starting components for the production of foamed or nonfoamed polyurethane plastics. To this end, the polyethers are reacted with polyisocyanates, optionally in stages and optionally in the presence of additives. In order to avoid any adverse effect upon the polyisocyanate polyaddition reaction and impairment of the properties of the resultant polyurethane plastics, the polyether polyols are generally required to be substantially free from chemical impurities.
On an industrial scale, polyether polyols are generally produced by the addition of alkylene oxides (particularly propylene oxide and/or ethylene oxide) to a compound containing active hydrogen atoms (for example, water, polyalcohols or polyamines) in the presence of a basic substance (generally alkali metal hydroxides) which acts as a catalyst. The conventional amount of catalyst is from about 0.1 to 1.0% by weight (from 1,000 to 10,000 ppm). On completion of the alkoxylation reaction, it is necessary, for the reasons explained above, to treat the polyether polyols in such a way that the concentration of basic substances in the end product is reduced to about 0.0005% (5 ppm) or lower (cf. U.S. Pat. No. 4,129,718).
In one of the known processes commonly used to reduce the concentration of basic substance in the polyether polyol, the basic catalyst (for example, KOH) is removed from the alkaline polymer in several steps. Initially, the polymer is neutralized (for example, by using dilute sulfuric acid). Most of the water which is present is then distilled off and the inorganic salts (for example, potassium sulfate) are crystallized. The precipitated salt is filtered off, the remaining water is then distilled off and the residual salt removed by filtration.
The disadvantages of the known neutralization processes are discussed in detail in the introduction of U.S. Pat. No. 3,833,669. The major problems encountered in such processes are difficulty in filtration due to the very finely divided particles of the salt and high consumption of energy and chemicals.
In another type of process for removing an alkaline catalyst from a polyether which has been used industrially, the catalyst is removed from the alkaline polymer by adsorption on a material such as synthetic magnesium silicate (see, U.S. Pat. No. 4,029,879). The disadvantages of this type of process (cf. U.S. Pat. Nos. 3,715,402 and 4,129,718) include the possibility that the product polyol may be damaged by oxidation and considerable expense due to high consumption of adsorbents and the relatively high cost of regenerating the adsorbent.